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Re: Bird reduce their "heating bills" in cold climates



--- On Mon, 7/5/10, Tim Williams <tijawi@yahoo.com> wrote:

> From: Tim Williams <tijawi@yahoo.com>
> Subject: Re: Bird reduce their "heating bills" in cold climates
> To: dinosaur@usc.edu
> Cc: tijawi@yahoo.com
> Date: Monday, July 5, 2010, 8:51 PM
> Jura <pristichampsus@yahoo.com>
> wrote:
> 
> 
> > You are oversimplifying here. "Saurischians" is
> really
> > "maniraptors and a couple of non-maniraptoran
> coelurosaurs.
> > The majority of known saurischian integument is scaly.
> 
> 
> 
> I don't see why this is a "one-or-the-other"
> situation.  Scaly integument and dino-fuzz could
> co-exist in coelurosaurs (and other dinosaurs).  After
> all, modern birds have scales on their feet and on part of
> the face.  

+++++++++++++++++++++++++++++++++++

There are some good evo-devo reasons to assume that the transition scales to 
feathers was an all, or nothing kind of affair (well, scales to filamentous 
stuff at least). The beta keratin gene for bird feathers is different from the 
beta keratin gene for scales. It's smaller, likely due to a deletion event (see 
Gregg 1984). 

Then there is the work of Zhou and Niswander (1996), and Sawyer & Knapp (2003) 
that show an antagonistic relationship between scales and feathers, with the 
scales on birds legs being a re-acquisition of the trait (by actively 
suppressing feather development) rather than being a hold over. This has 
further been supported by the discoveries of _Microraptor_, _Anchiornis_, 
_Pedopenna_ and a reassessment of the legs of _Archaeopteryx_ and 
_Confusciornis_ all of which showed feathers extending down to the feet, and 
suggesting that a "feathers all over" situation was the plesiomorphic 
condition. That birds have scales on their tarsal metatarsal region fits well 
with work by Alibardi & Thompson, showing that the scale formation on the this 
region of the foot occurs in a separate wave from that which covers the body.



The problem I have with Xu et al's "Tyrannosaurs were elephants" scenario is 
that they it requires an ontogenetic shift in integument, 
 it is the same as elephants losing hair as they reach adulthood, but it is 
nothing like that (furthermore, has anyone else noticed how not hairy baby 
elephants are?). It assumes that scales = skin, but they don't.  When a baby 
bird goes from soft down to rigid flight feathers, it is still just going from 
feathers to feathers. While such an integument shift is not out of the realm of 
possibility, it does require that these animals do something that no extant 
tetrapod does. 

Jason 

References
___________

Alibardi, L. and Thompson, M. 2001. Fine Structure of the Developing Epidermsis 
in the Embryo of the American Alligator (Alligator mississippiensis, 
Crocodilia, Reptilia). J. Anat. Vol.198:265-282.s

Gregg, K., Wilton, S.D., Parry, D.A., and Rogers, G.E. 1984. A Comparison of 
Genomic Coding Sequences for Feather and Scale Keratins: Structural and 
Evolutionary Implications. Embo J. Vol.3(1): 175-178.

Sawyer, R.H. and Knapp, L.W. 2003. Avian skin Development and the Evolutionary 
Origins of Feathers. J. Exp. Zool. (Mol Dev Evol). Vol.298B:57-72.

ZHOU, H. and NISWANDER, L. (1996) Requirement for BMP signaling in interdigital 
apoptosis and scale formation. Science 272: 738-41